Dispersion in time and space affect mating success and Allee effects in invading gypsy moth populations.

1. Understanding why invading populations sometimes fail to establish is of considerable relevance to the development of strategies for managing biological invasions. 2. Newly arriving populations tend to be sparse and are often influenced by Allee effects. Mating failure is a typical cause of Allee effects in low-density insect populations, and dispersion of individuals in space and time can exacerbate mate-location failure in invading populations. 3. Here we evaluate the relative importance of dispersal and sexual asynchrony as contributors to Allee effects in invading populations by adopting as a case study the gypsy moth (Lymantria dispar L.), an important insect defoliator for which considerable demographic information is available. 4. We used release-recapture experiments to parameterize a model that describes probabilities that males locate females along various spatial and temporal offsets between male and female adult emergence. 5. Based on these experimental results, we developed a generalized model of mating success that demonstrates the existence of an Allee threshold, below which introduced gypsy moth populations are likely to go extinct without any management intervention.

The influence of O3, NO2 and SO2 on growth of Picea abies and Fagus sylvatica in the Carpathian Mountains.

At 17 long-term pollution monitoring sites throughout the Carpathian Mountains, tree growth patterns and variation in growth rate were examined to determine relationship of tree growth to specific pollutants. Canopy dominant Picea abies and Fagus sylvatica were selected at each site. Basal area increment (BAI) values were calculated from raw ring widths and used as an estimate of tree growth. Across all sites, BAI chronologies were highly variable, therefore local conditions and forest structure accounted for considerable variation. Several significant relationships, however, implicated a role of pollutants on tree growth. Average levels (1997-1999) of NO(2) and SO(2) were inversely related to BAI means (1989-1999). Although average O(3) alone was not related to growth, the maximum O(3) value reported at the sites was negatively correlated with overall growth. A variable representing the combined effect of O(3), NO(2) and SO(2) was negatively correlated with both P. abies and F. sylvatica growth. Pollution data were used to categorize all sites into 'high' or 'low' pollution sites. Difference chronologies based on these categories indicated trends of decline in the 'high' pollution sites relative to 'low' pollution site. In the more heavily polluted sites, the BAI of Fagus sylvatica has declined approximately 50% and Picea abies has declined 20% over the past 45 years.

Spatial scale and the detection of density dependence in spruce budworm outbreaks in eastern North America.

Using two tests for direct density dependence and standard techniques of time series analysis, we identified density dependence in defoliation time series of the spruce budworm across its outbreak range in eastern North America over the years 1945-1988. We carried out analyses for the entire region and for grid cells of defoliation maps at five spatial scales created by aggregating the smallest grid cells. The rate of detection of direct density dependence, as assessed by two previously published methods, decreased with increasing spatial scale. Using both methods, density dependence was detected more frequently at the periphery of the outbreak range, where defoliation rate was lower. This result suggested that density-dependent regulation may be stronger in those areas. The first order autoregressive process was the basic model for defoliation dynamics overall and the most common model across spatial scales. Second-order processes were encountered much less frequently, and those commonly identified as resulting from delayed density dependence generally occurred across spatial scales at a rate expected by chance alone. Our results were similar to those of other published studies, which have found the detection of density dependence to decrease at larger spatial scales. The results also reinforced the importance of considering spatial scale when diagnosing population processes using time series of abundance for single species.

Effect of foliage proximity on attraction ofChoristoneura occidentalis andC. retiniana (Lepidoptera: Tortricidae) to pheromone sources.

Pheromone-baited traps located close to both host and nonhost crowns were more attractive than traps located between crowns for bothC. occidentalis Freeman andC. retiniana (Walsingham) at both 10 m and at 1.5 m above the ground. At 10 m height, traps located in host foliage were more attractive than those located in nonhost foliage, but at 1.5m height there was no significant difference. These results were obtained for both dense and sparse populations ofC. occidentalis and sparse populations ofC. retiniana. We conclude that the tree species on which a virgin female is located is not an important factor restricting mating between closely relatedChoristoneura spp. Also, the tree species on which a trap is located may not be an important factor that must be standardized in developing pheromone monitoring systems forC. occidentalis andC. retiniana.